The present invention relates to photographic silver halide coreshell emulsions. More particularly, the invention relates to a silver bromoiodide emulsion having grains comprising several phases with different iodide content, which emulsion shows better sharpness and sensitometric properties.
In recent years, there have been more strict requirements for silver halide emulsions for photographic use. This has increased the demands for the high level photographic characteristics such as, for example, high speed, excellent graininess, low fog, wider exposure latitude range and so on.
The above mentioned requirements have been satisfied by well-known silver bromoiodide grain emulsions having grains with a specific core-shell structure (in which an inner core is covered by one or more shell having different silver halide composition) and a high silver iodide content in the inner part thereof. It is well known in the photographic art that light absorbing increases in the order of silver chloride, silver bromide and silver iodide, but development activity correspondingly decreases in the same order. By using the above described core-shell silver bromoiodide emulsions, a good balance between light sensitivity and development activity has been obtained.
Examples of core-shell silver bromoiodide emulsion are described in many patent and literature references. For example, U.S. Pat. Nos. 4,668,614 and 4,728,602 describe a monodispersed coreshell silver bromoiodide emulsion having a core part comprising a silver iodide content of 10 to 45 mol % and a shell part comprising a silver iodide content lower than 5 mol %., with an average silver iodide content higher than 7 mol %. When this emulsion was examined by X-ray diffractometry, two peaks were evidentiated, the first one corresponding to the high iodide core part, the second one to the low iodide shell part. According to the claimed invention, the ratio between the diffraction intensity of the two peaks is preferably in the range of from 1/10 to 3/1, more preferably 1/3 to 3/1.
Similarly, European application EP 299,719 discloses a core-shell silver halide emulsion having a core comprising not less than 10 mol % of silver iodide, at least one shell consisting of silver bromide or silver bromoiodide, the outermost of which has a silver iodide content not higher than 5 mol %, and an average silver iodide content of not less than 10 mol %.
EP 309,119 discloses a core-shell silver halide emulsion having at least three silver bromide or silver bromoiodide phases of different composition. According to a preferred embodiment, the innermost phase has a silver iodide content of at least 10 mol %, the outermost phase has a silver iodide content of not more than 6 mol %, and the intermediate phase has a silver iodide content difference with the outermost or innermost phase of at least 3 mol %. When examined by X-ray diffraction, the claimed emulsion shows three or more diffraction peaks, each corresponding to a phase containing a different percentage of iodide.
EP 202,784 describes a core-shell silver halide emulsion having an inner core essentially consisting of silver bromide or silver bromoiodide and a plurality of shells. The outermost shell has a silver iodide content ranging from 0 to 10 mol %, the innermost shell has a silver iodide content at last 6 mol % higher than that of the outermost shell, and an intermediate shell has a silver iodide content is at least 3 mol % lower than that of the innermost shell and at least 3 mol % higher than that of the outermost shell.
U.S. Pat. No. 4,477,564 describes a multiphase silver bromoiodide emulsion having an average silver iodide content higher than 12%.
U.S. Pat. No. 4,614,711 describes silver bromoiodide grains with a core shell structure with a core of silver bromide or bromiodide and a first layer composed of silver bromoiodide, exterior to said core and containing more iodide than said core by 10 mol % or more.
U.S. Pat. No. 5,780,216 discloses a color negative silver halide photographic material with a core shell emulsion having a plurality of shells comprising an inner core consisting essentialy of silver bromide or bromoiodide and a plurality of shells of silver bromide or silver bromoiodide comprising a high iodide shell interposed between two shells consisting essentially of silver bromide.
U.S. Pat. No. 6,258,522 discloses a light-sensitive emulsion comprising silver bromoiodide grains of a core shell structure comprising: a) an inner core consisting essentially of silver bromide or silver bromoiodide with a silver iodide content within the range of 0 to 10 mole %, and b) a plurality of shells consisting essentially of silver bromide or silver bromoiodide, wherein a pure silver iodide phase representing from 0.1 to 5 mole % of the total silver halide grain content is interposed between two adjacent shells, and wherein at least one of said adjacent shells has a silver iodide content higher than 5 mole %.
Light-sensitive silver halide color photographic materials generally comprise a plurality of uniform layers of light-sensitive silver halide emulsions having different color sensitivities with dry thicknesses of a few microns coated on a support of cellulose triacetate or polyethyleneterephthalate. In such light-sensitive silver halide emulsions, there exist a large number of silver halide grains having grain sizes corresponding approximately to the wavelengths of visible rays and various crystal habits dispersed within a hydrophilic colloid such as gelatin having a refractive index smaller than that of silver halide. When visible light rays hit in the vertical direction the surface of such a light-sensitive silver halide emulsion layer, such rays are known to be scattered by the silver halide grains within the layer. This is said to be a phenomenon caused by the aforesaid grain sizes comparable to the wavelengths of visible rays and the difference in refractive index between grains and the binder. The extent of scattering may vary depending on the number of grains per unit volume, grains sizes, grain size distribution and the difference in refractive index between the silver halide grains and the binder. Thus, the visible light rays entering the light-sensitive silver halide emulsion layer, even when entering vertically the silver halide emulsion, is subject to scattering within the light-sensitive silver halide emulsion layer. Such a scattering causes the so called vagueness in the resulting image and consequently lowers its sharpness.
There have been proposed various methods to reduce the worsening of the image sharpness by light scattering as mentioned above.
One method of improving sharpness, disclosed at U.S. Pat. No. 4,312,941 and at U.S. Pat. No 4,391,884, involves the incorporation of a filter dye in a film layer between the exposing light source and a layer comprising a conventional grain light sensitive silver halide emulsion. The filter dye is made non diffusible either by means of a ballast group or by means of a mordanting material incorporated at a specific position in the film structure. Use of this spatial arrangement of absorber dye and emulsion reduces surface halation effects.
U.S. Pat. No. 4,439,520 discloses a radiation-sensitive high aspect ratio tabular grain silver halide emulsion comprised of a dispersing medium and silver halide grains, wherein at least 50 percent of the total projected area of the silver halide grains is provided by chemically and spectrally sensitized tubular silver halide grains having a thickness of less than 0.3 micrometer, a diameter of at least 0.6 micrometer, and an average aspect ratio of greater than 8:1.
U.S. Pat. No. 4,521,507 discloses a multi-layer light-sensitive silver halide color photographic material having a plurality of silver halide emulsion layers having different sensitivities to the different spectral regions of the visible light coated on a support, wherein at least 80% of the total number of silver halide grains in at least one said silver halide emulsion layers is represented by silver halide grains having grain sizes greater than 0.8 micrometer and grain size smaller than 0.65 micrometer.
These high aspect ratio silver halide emulsions, herein known as tabular grain emulsions, differ from conventional grain emulsions in many characteristics. One main characteristic is the relationship between the thickness and the equivalent circular diameter of the emulsion grains. Conventional emulsion grains tend to be isotropic in shape and, when incorporated in a layer structured film, tend to be randomly oriented within each layer. Tabular emulsion grains, tend to be anisotropic in shape and, when incorporated in a layer structured film, tend to align such that their major axis parallels the plane of the film base. This degree of anisotropicity is know as the emulsion aspect ratio (AR), typically defined as the ratio of the average grain equivalent circular diameter and the average grain thickness. The ability to control emulsion grain thickness and alignment within a film structure can allow the realization of otherwise unattainable degrees of photographic material performance.
xe2x80x9cResearch Disclosurexe2x80x9d, #25330, May, 1985, discloses methodologies related to the definition of particular arrangements of tabular grain emulsions having particular tabular grain thicknesses within a certain layer of a film structure so as to enable the attainment of specifically desired properties, such as speed or sharpness in underlying or overlying emulsion layers. No mention is made of the relationship between tabular grain thickness and the speed or sharpness of the emulsion layer comprising such grains.
U.S. Pat. No. 4,740,454 discloses a silver halide photographic material comprising a support having coated thereon at least one light-sensitive silver halide emulsion layer containing tabular silver halide grains which have an aspect ratio not less than 5:1 and which account for at least 50% of the total projected area of the silver halide grains present in the silver halide emulsion layer, the silver. halide photographic material further containing at least one 1,3,4-thiadiazole derivative.
U.S. Pat. No. 5,275,929 discloses a photographic material comprising a support bearing a photographic layer comprising a sensitized tabular grain silver halide emulsion of aspect ratio greater than 10:1 and of specific thickness to minimize the spectral reflectance in the region of the spectrum where the emulsion has its maximum sensitivity.
These methods have not been proved to be wholly satisfactory.
The present invention relates to a light-sensitive emulsion comprising silver bromoiodide grains of a core shell structure comprising: a) an inner core consisting essentially of silver bromide or silver bromoiodide and having a silver iodide content within the range of 0 to 10 mole %, and b) a plurality of shells consisting essentially of silver bromide or silver bromoiodide, wherein said silver bromoiodide grains exhibit an average grain size lower than 0.60 micrometer, an average grain thickness lower than 0.15 micrometer, and an average aspect ratio lower than 5:1.